This invention relates to a nonfreezing pipe and, more particularly, to a nonfreezing pipe for use as a water supply pipe extending from a pipe laid under the ground to a point of use over the ground or in the interior of architecture.
To avoid freezing of liquid flowing through pipelines, it has generally been carried out to insulate the pipelines subjected to the cold with heat insulation materials surrounding the external wall portion of the pipelines. However, these insulation materials and covering materials thereof are progressively deteriorated by changes in environmental conditions, resulting in breaking or lowering in insulating properties. For this reason, it is frequently required to repair or replace the heat insulation materials with new ones. Such a maintenance is expensive and takes a lot of time.
To solve such problems, it has been proposed to use a nonfreezing pipe comprising cylindrically shaped concentric inner and outer shells, the outer shell being tapered at its opposite ends and seam welded to the exterior wall portion of the inner shell at the opposites ends to form a double walled construction, the space between said inner and outer shells being evacuated of air to provide heat insulation (Japanese utility model application laying open No. 59-151971).
Such a nonfreezing pipe has excellent heat insulation properties so that it makes it possible to prevent the water from freezing. However, the nonfreezing pipe of the prior art has some serious problems awaiting a solution. For example, it is difficult to produce reliable nonfreezing pipes because of its construction. In general, the nonfreezing pipe is produced by a method comprising the steps of joining inner and outer shells at portions where tapered ends of the outer shell are in contact with the exterior wall portions of the inner shell to form a double walled pipe, evacuating a space between the inner and outer shells through a tip tube or suction port previously provided on or in the outer shell, and then closing the tip tube or sealing the suction port with a flap to complete a nonfreezing pipe. Because the double walled pipe is heated to an elevated temperature during evacuation, and because the inner shell is covered with the outer shell, the thermal expansion of the inner shell becomes smaller than that of the outer shell, resulting in generation of an excess stress acting on the joints between the inner and outer shells. Such an excess stress causes cracking in the joint at the time of cooling or aging of the resultant nonfreezing pipe.